It is common practice to provide an exhaust system for compartments, hoods, and other zones in which noxious gases and undesirable fumes are generated. Conventionally, the exhaust system includes one or more hooded compartments each of which is in communication with one end of an exhaust pipe through which a flow of exhaust gas is induced by means of a fan or blower. The exhaust pipe of each compartment may communicate with a gas conduit that is common to a plurality of compartments, or each compartment may have its own independent exhaust conduit. In either case, the rate at which the gas flows through the exhaust system must be sufficient to obtain effective exhaust of the noxious or otherwise undesirable fumes from each compartment so as to avoid contamination of the area adjacent that where the fumes are generated.
Currently, minimum flow of an exhaust gas stream is determined in a somewhat haphazard manner. For example, the exhaust stream inducing blower or fan has its speed adjusted from normal to less than normal, and frequently in successive stages, so as eventually to establish a minimum exhaust stream flow. One objection to this form of calibration is that substantial time is required to determine the minimum acceptable flow value.
Another disadvantage of the conventional calibration technique referred to above is that it is difficult or impossible to determine deterioration in the exhaust stream flow until the flow has been reduced to a level below that of the minimum value.
A further disadvantage of conventional systems is that the monitoring of less than all compartments coupled to a central exhaust system may compromise the safety of or disrupt work at one or more of the compartments that are connected to the central exhaust system.
An object of this invention is to overcome the objectionable characteristics referred to above.